It is desirable to determine the presence of objects, such as contraband, weapons, or explosives, that have been concealed, for example, in a moving vehicle, or, additionally, under the moving vehicle, in either case, without requiring the subjective determination of a trained operator. The determination should be capable of being made while the container is in motion. In case a detection is made, a visual image should be available for verification. The use of images produced by detection and analysis of penetrating radiation scattered from an irradiated object, container, or vehicle is the subject, for example, of U.S. Pat. No. 4,799,247 (Annis et al.) and U.S. Pat. No. 5,764,683 (Swift et al.), where are herein incorporated by reference. The techniques taught in the prior art, however, require that the motion of the inspected object relative to the source of radiation be at a controlled rate, either by moving the inspected object on a conveyor, by sweeping the orientation of the source, or by mounting both source and detector arrangement on a single movable bed and driving them past the inspected object at a known or determinable rate.
The use of x-rays traversing a moving railway car or other large shipping container has been taught in U.S. Pat. No. 5,910,973, issued Jun. 8, 1999, incorporated herein by reference. The '973 patent taught embodiments wherein transmitted x-rays are detected by one or more detectors placed on the side of the car distal to the source of irradiation. Disadvantages of the inspection systems based on transmitted x-rays include their typical insensitivity to organic materials having low attenuation, especially those in sheet form.